drm/dp: Don't attempt AUX transfers when eDP panels are not powered

If an eDP panel is not powered on then any attempts to talk to it over
the DP AUX channel will timeout. Unfortunately these attempts may be
quite slow. Userspace can initiate these attempts either via a
/dev/drm_dp_auxN device or via the created i2c device.

Making the DP AUX drivers timeout faster is a difficult proposition.
In theory we could just poll the panel's HPD line in the AUX transfer
function and immediately return an error there. However, this is
easier said than done. For one thing, there's no hard requirement to
hook the HPD line up for eDP panels and it's OK to just delay a fixed
amount. For another thing, the HPD line may not be fast to probe. On
parade-ps8640 we need to wait for the bridge chip's firmware to boot
before we can get the HPD line and this is a slow process.

The fact that the transfers are taking so long to timeout is causing
real problems. The open source fwupd daemon sometimes scans DP busses
looking for devices whose firmware need updating. If it happens to
scan while a panel is turned off this scan can take a long time. The
fwupd daemon could try to be smarter and only scan when eDP panels are
turned on, but we can also improve the behavior in the kernel.

Let's let eDP panels drivers specify that a panel is turned off and
then modify the common AUX transfer code not to attempt a transfer in
this case.

Tested-by: Steev Klimaszewski <steev@kali.org>
Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Tested-by: Eizan Miyamoto <eizan@chromium.org>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240202141109.1.I24277520ac754ea538c9b14578edc94e1df11b48@changeid

Author: dianders

drivers/gpu/drm/display/drm_dp_helper.c

@@ -532,6 +532,15 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 
 	mutex_lock(&aux->hw_mutex);
 
+	/*
+	 * If the device attached to the aux bus is powered down then there's
+	 * no reason to attempt a transfer. Error out immediately.
+	 */
+	if (aux->powered_down) {
+		ret = -EBUSY;
+		goto unlock;
+	}
+
 	/*
 	 * The specification doesn't give any recommendation on how often to
 	 * retry native transactions. We used to retry 7 times like for
@@ -599,6 +608,29 @@ int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset)
 }
 EXPORT_SYMBOL(drm_dp_dpcd_probe);
 
+/**
+ * drm_dp_dpcd_set_powered() - Set whether the DP device is powered
+ * @aux: DisplayPort AUX channel; for convenience it's OK to pass NULL here
+ *       and the function will be a no-op.
+ * @powered: true if powered; false if not
+ *
+ * If the endpoint device on the DP AUX bus is known to be powered down
+ * then this function can be called to make future transfers fail immediately
+ * instead of needing to time out.
+ *
+ * If this function is never called then a device defaults to being powered.
+ */
+void drm_dp_dpcd_set_powered(struct drm_dp_aux *aux, bool powered)
+{
+	if (!aux)
+		return;
+
+	mutex_lock(&aux->hw_mutex);
+	aux->powered_down = !powered;
+	mutex_unlock(&aux->hw_mutex);
+}
+EXPORT_SYMBOL(drm_dp_dpcd_set_powered);
+
 /**
  * drm_dp_dpcd_read() - read a series of bytes from the DPCD
  * @aux: DisplayPort AUX channel (SST or MST)
@@ -1858,6 +1890,9 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 	struct drm_dp_aux_msg msg;
 	int err = 0;
 
+	if (aux->powered_down)
+		return -EBUSY;
+
 	dp_aux_i2c_transfer_size = clamp(dp_aux_i2c_transfer_size, 1, DP_AUX_MAX_PAYLOAD_BYTES);
 
 	memset(&msg, 0, sizeof(msg));

drivers/gpu/drm/panel/panel-edp.c

@@ -413,6 +413,7 @@ static int panel_edp_suspend(struct device *dev)
 {
 	struct panel_edp *p = dev_get_drvdata(dev);
 
+	drm_dp_dpcd_set_powered(p->aux, false);
 	gpiod_set_value_cansleep(p->enable_gpio, 0);
 	regulator_disable(p->supply);
 	p->unprepared_time = ktime_get_boottime();
@@ -469,6 +470,7 @@ static int panel_edp_prepare_once(struct panel_edp *p)
 	}
 
 	gpiod_set_value_cansleep(p->enable_gpio, 1);
+	drm_dp_dpcd_set_powered(p->aux, true);
 
 	p->powered_on_time = ktime_get_boottime();
 
@@ -507,6 +509,7 @@ static int panel_edp_prepare_once(struct panel_edp *p)
 	return 0;
 
 error:
+	drm_dp_dpcd_set_powered(p->aux, false);
 	gpiod_set_value_cansleep(p->enable_gpio, 0);
 	regulator_disable(p->supply);
 	p->unprepared_time = ktime_get_boottime();

drivers/gpu/drm/panel/panel-samsung-atna33xc20.c

@@ -72,6 +72,7 @@ static int atana33xc20_suspend(struct device *dev)
 	if (p->el3_was_on)
 		atana33xc20_wait(p->el_on3_off_time, 150);
 
+	drm_dp_dpcd_set_powered(p->aux, false);
 	ret = regulator_disable(p->supply);
 	if (ret)
 		return ret;
@@ -93,6 +94,7 @@ static int atana33xc20_resume(struct device *dev)
 	ret = regulator_enable(p->supply);
 	if (ret)
 		return ret;
+	drm_dp_dpcd_set_powered(p->aux, true);
 	p->powered_on_time = ktime_get_boottime();
 
 	if (p->no_hpd) {

include/drm/display/drm_dp_helper.h

@@ -464,9 +464,15 @@ struct drm_dp_aux {
 	 * @is_remote: Is this AUX CH actually using sideband messaging.
 	 */
 	bool is_remote;
+
+	/**
+	 * @powered_down: If true then the remote endpoint is powered down.
+	 */
+	bool powered_down;
 };
 
 int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset);
+void drm_dp_dpcd_set_powered(struct drm_dp_aux *aux, bool powered);
 ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 			 void *buffer, size_t size);
 ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,